The normal menstrual cycle
To understand PCOS you need to understand the normal menstrual cycle.
The female reproductive system
Women have two ovaries, located in the pelvis alongside the uterus. Their main functions are to release eggs and produce hormones. At birth, the ovaries are provided with approximately one million eggs, each surrounded by cells which develop into a small fluid filled blister known as a follicle.
The hypothalamus in the base of the brain signals the pituitary gland to secrete FSH (follicle-stimulating hormone) and LH (lutenizing hormone). In the first half of the menstrual cycle, these pulses are infrequent and FSH is secreted preferentially, allowing a crop of follicles to grow inside the ovaries. This phase is therefore called the "follicular" phase. The follicles secrete estrogen and acquire receptors for LH. As estrogen levels rise, the hypothalamus pulses more and more frequently. estrogen stimulates the growth of the breasts and the lining of the uterus. Most of the follicles degenerate in the next few days and only one is mysteriously selected. This dominant follicle (the "egg") matures over the next few days and just before ovulation - when the dominant follicle is expelled from the ovary - estrogen levels peak.
After ovulation, the dominant ruptured follicle becomes the corpus luteum. This little body secretes progesterone, the hormone that promotes the growth of blood vessels of the uterine lining in preparation of the implanting of a fertilized egg. This phase is called the luteal phase. If the egg is not fertilized, estrogen and progesterone levels fall, and the uterine lining is shed and menstruation occurs. At the time of puberty when girls have reached a critical muscle-to-fat ratio, the hypothalamus gives the GO signal for the start of menstruation. Excessive weight loss, exercise or stress can negate that signal. At the peak years of reproductive life most women have a regular 28-day cycle, plus or minus 2–3 days. The first day of the menstrual period is day 1. Ovulation usually occurs on day 14 to 15, but can occur anywhere from day 9 to 17 due to variability in the length of the follicular phase of the cycle. Around menarche and menopause, the cycles are irregular and often not associated with ovulation.